Charge distribution in series combination of capacitors

I think that you should forget about categorizing this arrangements of plates as being in parallel or in series and look at the problem in a different way.

Calling the plate on the left $1$, the potentials of plates $1$ and $3$ are the same and they are electrically isolated.
This tells you something important about the magnitude of the electric field between the plates $1$ and $2$, and between $2$ and $3$.
From this you can get the relative distribution of charges on the plates and hence solve the problem.

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Another way is to find the distribution of charges which minimises the electric potential energy of the system of charges on the plates.